Power systems, such as those used in propulsion, have a fuel combustion driven prime mover, such as a diesel internal combustion engine, as the main source of power. In a traditional diesel engine, such as those used in marine propulsion, there is a direct connection between the engine and the motors that drive the propellers. One or two such engines are generally used, depending on the size of the vessel. The problem here is that these engines operate under constant rotational speed, irrespective of the speed of the vessel. Hence, with low speed, the energy consumption is high compared to the actual output, leading to high levels of CO2 emission, high energy consumption and high maintenance costs. In addition, breakdown of one component may quite often lead to total engine damage.
The above problem is obviated in case of a diesel-electric propulsion system. Herein, one or two large main engines may be substituted by more and smaller diesel engines, each of them being connected to an electrical generator delivering the electricity to a main switch-board, which is an AC distribution bus (AC: alternating current), split into two by a disconnector or a bus tie. The main switch-board supplies power to multiple propeller motors via respective rectifiers and inverters. The advantage of such a system is the use of sufficient numbers of smaller diesel engines, i.e., the usage of all engines at all times is avoided. If the engine is working idle or the vessel is moving with reduced speed, its may be sufficient to connect only one generator, which in return may be operated on optimum capacity and efficiency. On the other hand, a higher power demand may call for high speed and running of all of the generators. This flexibility gives substantial energy-savings, at the same time securing optimal output effect.
However, such diesel electric power systems suffer from a few drawbacks. For example, the main switch-board, which is an AC distribution bus, is rated for high current levels and represents a high cost element. Also, for optimal operation, at least one generator on each bus must be in operation. This results in a considerable reduction in fuel consumption. In uncritical operations the operation with a closed bus tie is possible. In these cases only one generator may be operated at low load. However, in case of the need of higher capacity, another generator must be switched on to the main switch-board. To do so, a normal synchronizing procedure must be followed, which increases the start-up time of the generator sets. Furthermore, the selectivity between generators in case of failure is a difficulty in many applications.